This invention relates to polyetherimides, and more particularly to polyetherimide compositions having improved stress-crack resistance. Still more particularly, the polyetherimide compositions of this invention are polymer blends containing, in addition to a polyetherimide, an acrylate core-shell type rubber and a polycarbonate. These blends are characterized by remarkably improved environmental stress-crack resistance and an excellent balance of mechanical properties.
Polyetherimides are widely accepted for use because of their good strength properties at elevated temperatures, electrical properties, and for their heat resistance. Even though these resins are resistant to a wide range of chemicals, they are susceptible to crazing and stress cracking when exposed to solvents and fluids commonly encountered in coating, electronic and automotive applications. Polyetherimides also are somewhat lacking in impact strength and ductility; when molded parts encounter such solvents they may become crazed or stress-cracked and subject to catastrophic failure in use.
Improvement in environmental stress-crack resistance thus becomes needed for better performance of articles fabricated from polyetherimides, particularly for use in applications such as automotive and electrical parts where the articles may be subject to attack by solvents and fluids. Of course, it is also important that such improvement be achieved without substantial adverse effects on desirable properties of the polyetherimide. The acceptance of polyetherimides for many applications over other resins by the compounding art depends on the resin's excellent dimensional stability at elevated temperatures. Methods for increasing resin environmental stress-crack resistance at the expense of impact strength and rigidity at elevated temperatures would likely not be readily adopted by the industry.
A variety of impact modifiers are known for use with thermoplastics. Generally, these modifiers are rubbery compositions that can be blended or incorporated into the thermoplastics. Very few rubbery modifiers have been found to be effective for improving impact in polyetherimide resins, and many of those that do often cause adverse affects, including losses in other key properties.
Ternary blends of polyetherimides with polycarbonates and acrylic rubber interpolymers are known in the art and have been shown to have improved impact strength. For example, in U.S. Pat. No. 4.673,708, there are disclosed blends comprising from about 25 to about 33 wt. % of the polycarbonate component based on the total weight of the blend, together with minor amounts of the rubber component. Although the blends are disclosed to have improved impact strength, little improvement is shown by the reference for ternary blends containing less than 30 wt. % polycarbonate.
Thus, there continues to be a need for high temperature thermoplastic polyetherimides with improved resistance to environmental stress cracking, and for a method to improve the environmental stress cracking resistance of polyetherimides without substantially reducing the desirable balance of mechanical properties that characterize these resins.